1. Field of the Invention
This invention relates to intercom systems and, more particularly, to intercom systems employing automatic squelch circuits for use in small aircraft or other high noise environments.
2. Description of the Related Art
In a typical small aircraft intercom system, several different audio channels are controlled and combined to form output audio to be heard by users. For example, audio channels are typically provided to allow the pilot, co-pilot and passengers to communicate with one another through microphone inputs. External communications through a communications radio are further accommodated to allow communications between, for example, an air traffic controller and the pilot and/or co-pilot. Similarly, an interface for connection to a telephone may be provided. Audio associated with additional aspects of the control of the aircraft, such as altimeter warnings and marker beacon signals, may also be integrated within the intercom system. Finally, the aircraft intercom system may accommodate music inputs to allow the pilot, co-pilot and/or passengers to listen to music.
During flight, the cabin of a small aircraft is typically quite noisy, due to both engine noise and wind noise. Accordingly, the occupants of the aircraft cabin typically wear headset units having headphones and microphones. Rather than leaving microphones on continuously, squelch circuits are typically provided to activate selected microphones upon the presence of a user""s voice. This advantageously prevents unwanted aircraft cabin noise picked up by the microphones from being introduced into the intercom system when voice audio is not detected.
The squelch circuits of many aircraft intercom systems have manual controls which must be set by the user. The manual squelch control is used to set the threshold level at which an input signal must rise before the microphone signal path is activated. As the noise in the aircraft cabin increases or decreases, corresponding adjustments to the squelch control may be necessary to maintain proper operation of the intercom system. Unfortunately, the noise in the aircraft cabin may vary considerably during a given flight, thus requiring multiple adjustments to the manual squelch control.
Due to the varying types of audio handled by a typical aircraft intercom system, various user operated controls in addition to the manual squelch control are also provided to select various modes of operation. In general, it is desirable that apparatus for use by pilots and co-pilots in an aircraft be as simple as possible to use while allowing flexibility. Particularly for safety, it is further desirable to reduce the number of controls which must be manually operated.
Thus, some aircraft intercom systems utilize automatic squelch circuits. In such systems, as changes in the noise level in the aircraft cabin occur, the threshold for activating a corresponding microphone path is automatically re-adjusted. This thus allows simplified operation.
However, various aircraft intercom systems having automatic squelch circuits are believed to suffer from a number of drawbacks. For example, it is believed that some intercom systems having automatic squelch controls do not operate optimally in that they may be prone to relatively long delays when voice is initially presented, thus clipping the initial speech of a user. Similarly, these systems may further turn off prematurely, thus clipping the middle or end portions of normal speech.
Additionally, some systems are not associated with good repeatability, in terms of consistent operation with varying operating conditions. That is, as operating conditions including temperature, power supply voltage, and background noise vary, the operation of some systems may be adversely affected.
Finally, some systems require a relatively large number of components for implementation. This may result in increased cost and/or power consumption. Furthermore, the requirement of a relatively large number of components may further inhibit compact design, which is another important feature for a small aircraft intercom system.
Therefore, it would be desirable to provide an automatic squelch for an intercom system used in small aircraft or other high noise environments which minimizes unwanted clipping of speech. It would further be desirable to provide an intercom system which operates consistently, which may be implemented using a relatively small overall number of components, and which consumes relatively low power.
Various problems described above are in large part solved by an intercom system for use in a small aircraft or other high noise environment including an automatic squelch in accordance with the present invention. In one embodiment, an automatic squelch is provided within an intercom to control the gating of a microphone signal path. The intercom system may include various audio inputs including inputs for receiving microphone signals from the pilot, co-pilot and passengers, an input for receiving aircraft radio audio, and a music input. The intercom system may selectively combine and amplify the audio inputs to form audio output signals that drive the headphones of users. The automatic squelch includes an analog-to-digital converter coupled to receive an analog microphone input signal which is configured to convert the analog microphone input signal into a digital input signal. A peak detector is further provided to maintain a peak value indicative of peaks in the digital input signal. The peak value maintained by the peak detector reacts relatively quickly to changes in the digital input signal. In one implementation, when the digital input signal is larger than the peak value, the peak value is set at the level of the digital input signal. When the digital input signal is less than the peak value, the peak value is xe2x80x9cdecayedxe2x80x9d or reduced at a relatively fast decay rate. A background noise detector is further provided for maintaining a background value indicative of background noise in the digital input signal. In one implementation, the background value is adjusted by comparing the peak value to the background value. When the peak value is greater than the background value, the background value is increased at a relatively slow attack rate. When the peak value is less than the background value, the background value is decreased at a relatively slow decay rate. The background noise detector thus varies the background value relatively slowly in response to changes in the peak value. A comparator is further provided to determine whether the peak value exceeds a predetermined threshold value. In one implementation, the predetermined threshold value is obtained through a table lookup function wherein the background value is used as an index to index into a lookup table including a plurality of corresponding threshold values. If the peak value exceeds the threshold value, a one-shot timer is triggered to activate a microphone signal path. In one implementation, hysteresis is further provided to lessen the threshold required to retrigger the one shot timer while it is active. The functionality of the peak detector, background noise detector, and one shot timer may be implemented by software code executed within a microcontroller.
An intercom system employing the improved automatic squelch may advantageously minimize unwanted clipping of speech. Additionally, the improved automatic squelch may result in consistent operation regardless of varying operating conditions, may be implemented using a relatively small number of components, and may consume relatively low power.